Healthy gait exhibits natural stride-to-stride fluctuations that over large series of observations reveal a particular order or structure. This phenomenon is referred to as physiological complexity. This complexity is thought to be beneficial to the locomotor system as it allows for optimal adaptability, while also ensuring stable, coherent movement, both of which are required to safely navigate realworid, dynamic environments. Walking to an invariant external stimulus as it is the case in current gait rehabilitation leads to a breakdown of this critical property of gait, as it contains no complexity and is purely periodic. We propose a novel variable visual stimulus that reflects the natural variability observed in healthy gait. The stimulus is based on the theory of fractals, meaning it is not simply a randomly variable stimulus. Rather, it has a certain repeated pattern contained within a varying temporal structure, mirroring healthy gait. We propose that this design provides optimal stability and flexibility to the neuromuscular system. In Aim 1, we will carry out an experiment in older adults to consolidate the theory that complexity in gait predicts a person's ability to adapt to internal and external perturbations. In Aims 2 and 3, we will determine the short- and long-term effects of walking in time to the variable visual stimulus on the gait of older adults who are at risk of falls due to poor gait performance. Our preliminary data has shown that the complex structure of gait variability is altered in older adults, but can be restored to levels similar to young adults when they are exposed to a variable external stimulus. We aim to investigate the long-term and retention effects of training with this stimuli (compared to training with an invariant or a completely random visual stimulus) on the restoration of optimal gait variability. We expect that our findings will confirm the efficacy of the proposed variable visual stimulus by positively influencing falls risk and function in older adults, thereby revealing a simple, low-cost therapy that could potentially enable them to remain independent for longer. The longer term goal of our research is. to apply this novel stimulus, and the innovative theoretical framework on which it is based, to all populations that experience gait impairments due to injury (e.g. post-operatively) or disease (e.g. stroke).